PARTNER FEATURE: Open virtual 5G Radio Access Networks (RANs) have been receiving a lot of attention lately as two of the big four mobile equipment vendors have announced that they now support open virtual RAN solutions. This is welcome news to carriers who want to see more competition and flexibility in the 5G RAN.

Open solutions to date have relied on Commercial Off-The-Shelf (COTS) hardware, such as standard x86 CPUs and standard Ethernet Network Interface Cards (NICs). Work by organisations like the O-RAN Alliance and Telecom Infra Project (TIP) is now resulting in specifications for both open interfaces and whitebox implementations that can address the performance requirements of 5G RAN both now and in the future.

One area that has been improved significantly in whitebox implementations is time synchronisation. More stringent requirements for end-to-end latency and increased antenna and base station density means that timing synchronisation and accuracy has become a critical capability that has not been fully available on COTS hardware until now.

While there are many COTS hardware offerings that support IEEE 1588v2 Precise Time Protocol (PTP), 5G requires support of time synchronisation accuracy of minimum 65ns for carrier aggregation and similar use cases. To ensure this level of precision, both PTP and precise frequency synchronisation technologies like Synchronous Ethernet are recommended to ensure synchronisation in both the physical and protocol layer. This will be even more essential as we move to higher frequency radio spectrums like millimetre wave (mmWave) with large MIMO antenna configurations.

An active member of both the O-RAN Alliance and TIP, Silicom is one of the few companies with experience of developing and delivering open 5G virtual RAN solutions to global telecom carriers. Silicom solutions based on Intel technology include Central Unit (CU) and Distributed Unit (DU) whitebox appliances, which will be crucial volume components in 5G virtual RAN networks. Being a pioneer has provided some valuable insights into what is required both today and in the future for open virtual solutions to be successful.

Silicom is currently engaged in solution evaluations with Tier 1 carriers across the globe.  Carriers have shown a tremendous interest in open solutions based on COTS hardware and are active members of the O-RAN Alliance and TIP. Nevertheless, carriers have stringent requirements that have to be met, especially for time synchronisation. Carriers also have unique requirements depending on their existing infrastructure, services to be supported and the trade-offs that need to made in meeting their business objectives. This means that solutions based on COTS hardware also need to be flexible and adaptable to different carrier situations.

One of the aspects of 5G that is often left unstated is its broad scope. It will take several years, if not more than a decade, to realise all the solutions and potential that 5G is designed to offer. This inevitably means that 5G solutions need to be able to meet current requirements as well as future requirements that are still emerging.

For example, current deployments of 5G are what are termed “non-stand-alone,” as they still rely on the 4G LTE core, but 5G “stand-alone” deployments are just around the corner, leading to new demands. This will include more network slicing implementations using intelligent controllers, like the “near-real-time” and “non real-time” controllers defined in the O-RAN Alliance architecture. New radio interfaces like 5G mmWave are being deployed, leading to new requirements. Many of the services driving the design and architecture of 5G, such as massive industrial IoT and ultra-reliable low latency services, have yet to be deployed at scale.

This means that a great deal of flexibility and adaptability needs to be built into 5G RAN products in order to accommodate future unforeseen requirements. The 5G architecture, and in particular the RAN architecture, has been designed with flexibility in mind. Organisations like O-RAN Alliance and TIP have added extra value by defining the open interfaces and controllers needed to deploy open virtual RAN solutions. However, underneath this layer there is still hardware on which these virtual software solutions are deployed and this is where the flexibility to adapt to different needs and circumstances is critical.

For example, to take maximum advantage of the flexibility that virtual CU and DU implementations provide, it must be possible to deploy these functions at various locations in the RAN. The DU can be close to the Radio Unit (RU) and antenna at the edge of the RAN or it can be located centrally together with the CU close the Mobile Edge Cloud or at any point in between. For that function to operate properly, the hardware supporting it must have the appropriate computing power, storage, RAM and acceleration and offload technology to ensure seamless performance.

It is these insights that drove Silicom to develop a broad portfolio of whitebox and NIC solutions based on Intel technology that can not only accommodate various deployment scenarios, but can also adapt to carrier preferences with respect to which software and hardware components they need.

Silicom provides a full whitebox implementation of an open virtual RAN CU and DU that can be adapted and customised to meet customer needs. For example, the whitebox is delivered with Silicom CU and DU virtual reference software, but this can be replaced with another vendor’s software, like Intel© FlexRAN reference solution. In addition, the whitebox appliance hardware can be customised to accommodate more powerful CPUs or additional CPU nodes, additional RAM or storage. All of Silicom’s whiteboxes are powered by Silicom Network Interface Cards (NICs), which are also provided with a broad range of options, ranging from standard Ethernet NICs to FPGA acceleration NICs that can fully or partially offload CU and DU functionality.

One differentiating feature of Silicom solutions is time synchronisation support. Silicom provides full support for IEEE1588v2 PTP with the telecom profile extensions to PTP defined by ITU-T in G.8272, G.8273 and G.8275 standards. This enables Silicom whiteboxes to act as Telecom Grandmaster (T-GM), Boundary (T-BC) and Time Slave (T-SC) clocks in the network. Silicom whiteboxes can also act as Primary Reference Time Clocks (PRTC Class B), which has up to now only been available in switches and dedicated time sync equipment, providing a maximum time error of 40 nanoseconds.

When these PTP capabilities are combined with Synchronous Ethernet, a wide range of time synchronisation deployment options can be supported with extremely high accuracy. Silicom whitebox and Ethernet NIC solutions provide unique support for a complete Synchronous Ethernet solution according to ITU-T G.8261. This includes deployment as a Synchronous Ethernet Equipment Clock (G.8262 EEC) with Ethernet Synchronisation Message Channel (ESMC) according to G.8264. This allows synchronisation both at the protocol layer using PTP and the physical layer using Synchronous Ethernet where the PTP Telecom Boundary Clock (T-BC) uses the Synchronous Ethernet Equipment Clock (EEC) for syntonisation.

Combining PTP and Synchronous Ethernet results in lower Maximum Time Interval Errors (MTIE) where Silicom has been able to show holdover of a few hours performance at 1.5 microseconds MTIE.

Comprehensive time synchronisation support also leads to rationalisation in the RAN, leading to fewer devices that need to be deployed. For example, if multiple RUs need to be connected to a DU, a Fronthaul Gateway switch is required as it has the necessary time synchronisation capabilities. However, with the Silicom DU whitebox support for a broad range of time synchronisation options, it is possible to remove the Fronthaul Gateway and aggregate RUs directly in the DU. This can lead to significant savings in the RAN.

The availability of highly accurate time synchronisation capabilities means that open solutions based on COTS hardware are now viable both from a performance and cost perspective. When this is combined with the ability to extend and customise whitebox hardware to meet specific requirements, it provides carriers with a solution that can meet their needs today and can be adapted to meet their needs in the future.

The deployment of 5G has just begun, with more demanding services and requirements expected to be supported in the future. Carriers today are exploring whitebox and NIC products to implement open virtual RAN solutions in their network. Many of these trials are software-based running on Intel based X86 appliance with time synchronisation Ethernet NICs. But there are also FPGA NICs for acceleration of Forward Error Correction (FEC) functions as well as layer 1 offload of the open fronthaul interface between the DU and the Radio Unit according to the 7.2 split defined by the O-RAN Alliance. As more demanding services are deployed, more offload and acceleration is anticipated to meet performance demands.

With the implementation of comprehensive time synchronisation support in the open virtual RAN whitebox developed by Silicom, one of the final hurdles for open COTS hardware to achieve the required performance levels necessary for 5G deployment has been removed. Carriers can now consider COTS-based open virtual RAN solutions a viable option for their current and future 5G deployment needs.